The rapid increase in capacity of on-chip memories in recent years has renewed the search for a universal embedded memory technology that combines fast read/write, low voltage operation, low power consumption, non volatility, infinite endurance and compatibility with CMOS processes. Spin-transfer torque magnetoresistive random access memories (STT-MRAM) have been recognized as promising candidates since their inception. The technology is innately non-volatile, and it has been shown that STT-MRAM cells based on perpendicularly magnetized Magnetic Tunnel Junction devices (PMA-MTJs) can be written at high speed with low power. FIG. 1 is a diagram of a cross section of a spin-torque-transfer magnetic tunnel junction device of the related art. “Demonstration of fully functional 8 Mb perpendicular STT-MRAM chips with sub-5 ns writing for non-volatile embedded memories,” January, et al., Digest of Technical Papers of the 2014 Symposium on VLSI Technology (VLSI-Technology), pp. 1-2, June 2014, found Mar. 11, 2015 at: ieeexplore.ieee.org/stamp/stamp. jsp?tp=&arnumber=6894357&isnumber=6894335, describes a PMA-MTJ stack is based on an alloy of cobalt, iron and boron (CoFeB) free layer 20 sandwiched between a capping layer 15 and a magnesium oxide (MgO) tunnel barrier 25 providing the perpendicular anisotropy. The tunnel barrier 25 is formed on a reference layer or pinning layer 30 of the CoFeB alloy. The reference layer 30 is formed on a bottom electrode 35. The bottom electrode 35 is for external connection to external devices. A hard mask contact layer 10 is formed on the capping layer 15 and the top electrode 5 is formed on the hard mask contact layer 10.
Recent reports show that MTJ devices can be operated at sub-nanosecond switching times that is as fast as SRAM and that the MTJ device size can be smaller than 25 nm and cell size about 6 F2 (minimum feature size for an integrated circuit technology). This cell size is as small as stand alone DRAM. Moreover, MTJ device is non-volatile (similar to Flash memory) and has much higher endurance than Flash memory. However, while MRAM is very versatile, it is challenging to design an MTJ stack that would excel in all performance requirements. One approach would be to use different stacks for different applications, but the cost of integration would increase dramatically.